Hi Bill, Lots of resources out there on EEVblog forums and elsewhere. One nice alternative to the avalanche pulser is a very fast comparator like the ADCMP580 from Analog Devices. Generate a pulse of whatever width you want with slower gear, and have the comparator sharpen the edges.
See here: http://www.eevblog.com/forum/testgear/inexpensive-diy-50ps-ultra-fast-pulse-generator/ and http://www.starlino.com/build-a-really-fast-pulse-generator-50ps-rise-time-using-an-ultra-fast-sige-comparator.html For a couple of examples. The EEVblog thread also has a link to info on an avalanche pulser using a transistor that avalanches at 30V rather than 70+. -Logan On Tue, Apr 12, 2016 at 2:28 PM, Alan Melia <alan.me...@btinternet.com> wrote: > Bill, Avalanche pulse gens only require high voltage because of the high > VBRcbo and the gain of normal NPN transistors. I cant find the reference > now it might have been a 1970s Ham Radio but if you use the same circuit as > Jim but put an NPN "upside down" that is emitter where the collector is in > Jim's circuit you can fire off fast pulses from a 12v supply, instead of > requiring 70 to 100v. I do wish I could locate the source as I have had > several arguments about it :-))....in the nicest possible way of course. > > Alan > G3NYK > > ----- Original Message ----- From: "BIll Ezell" <w...@quackers.net> > To: <time-nuts@febo.com> > Sent: Tuesday, April 12, 2016 9:30 PM > Subject: [time-nuts] Fast risetime pulse generator > > > > (cross-posted to volt-nuts) >> After paying only limited attention to this topic, I suddenly have a need >> for a pulse generator that has <150 ps risetime and a pulse width of at >> least 2 ns. 100mv amplitude or more is fine. I've looked at the classic Jim >> Williams avalanche generator, but I don't want to have to deal with the >> (relatively) high voltage source needed. >> >> I've done microwave design using Gunn diodes, so I'm drawn to using a >> step-recovery diode. The topology seems very straightforward, and I can >> build it right onto a BNC connector, no PCB. >> >> I'm thinking using an SMD835 diode, biased at ~1ma. The (sketchy) >> datasheet claims a T of 20 nsecs and a Tr of 85 ps, Cj of 0.4 to 0.8 pf. >> >> Questions: >> >> The obvious, is it reasonable? >> >> Is the bias current reasonable? I'm assuming the bias current is actually >> dependent on the repetition rate, you need enough current to replenish the >> charge within one pulse cycle. I suppose I could compute it from the stated >> junction capacitance, but I'm not sure that's the only factor. >> >> Will the stored charge actually give me the desired transition rate into >> 50 ohms? Hmm, again I should be able to compute this, but any other factors >> ignoring the non-diode ones like cap inductance? >> >> How should I compute the coupling cap from the diode to the load? Use the >> impedance at the pulse rep rate? Seems reasonable. BTW, I don't care about >> droop in the pulse, just the risetime. (measuring overshoot in an HF >> amp). Again, just want to verify that the obvious answer is the correct >> one. I clearly need to be very careful about the inductance. >> >> Thanks, Bill >> >> -- >> Bill Ezell >> ---------- >> The day Microsoft makes something that doesn't suck >> will be the day they make vacuum cleaners. >> Or maybe Windows 10. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.